Description of the Prior Art
In a contact printing electrostatic imaging system producing one or more colors, each color having its own station, paper is passed over a charging head containing a large number (for example, in one system approximately 13,000) of electrodes (or nibs). Negative charges are placed at selected locations on the paper by selected electrodes in accordance with the location of an image to be applied. The paper is then passed over part of a curved side of a rotating applicator roll covered with toner solution. (The paper passes over the curved side of the roll while the roll is turning in a direction opposite to the motion of the paper.) The rotating applicator roll has previously been covered with a bath of positively charged toner particles (colored pigment) suspended in a clear isopar (isoparaffinic material). The charged pigment is attracted to the oppositely charged areas, forming a latent image, on the paper as the paper passes over the applicator roll. At locations where toner pigment is neutralized (positive meets negative) the pigment is retained and the latent image is developed. The surface of the paper carrying the toner solution is quite wet at this point. The undeveloped toner solution is scavenged from all areas of the paper by means of a vacuum applied to the paper by a vacuum channel. From the vacuum channel the paper passes to the next station, if any, where another color is applied to the paper.
In forming a color image, a total of four stations are required to apply four different colors to the paper in order to produce the full spectrum of colors.
In the prior art, the applicator roll has parallel spiral grooves milled into its surface, each groove spirally encircles the roll with a predetermined pitch along the length of the roll. The grooves occupy a selected percentage (typically 50%) of the circumferential surface of the roll. Each groove acts as a reservoir supplying toner solution to the paper, encouraging a maximum density development of the latent image. Once the paper contacts the surface of the roll nearly all of the toner solution is removed from the surface of the roll. However, some "old" (pigment depleted) toner still remains in the grooves. It is desirable to remove and/or refresh the "old" toner solution in the grooves to promote maximum image development and to uniformly and fully develop images.
In the prior art, the grooves in the applicator roll are purged of "old" toner solution and the surface, including the grooves, of the roll are recoated with new toner solution, as a result of toner solution being forcibly directed against the underside of the turning applicator roll which is located within a toner station tank.
The toner solution is forced against the underside of the roll through a slot orifice manifold assembly. The manifold assembly is a rectangularly shaped distribution channel having an elongated slot along its top through which toner solution is forced against the roll under pressure from a large external pump. The pump circulates toner solution from a toner solution reservoir bottle through an aspirator to the slot orifice manifold assembly and onto the applicator roll. Excess toner solution then falls from the roll or the paper into the tank and then is routed by tubing back to the toner solution reservoir bottle. The slot orifice manifold assembly also contains a slotted distribution tube to assist in equalizing the pressure along the length of the manifold slot.
Flow of toner solution through the aspirator creates a vacuum which is connected by tubing to the vacuum channel which assists in removing the last remnants of undeveloped toner solution from the paper, as the paper passes over the vacuum channel.
The circulating pump is sized to impinge toner on the bottom of the applicator roll at a high enough pressure to displace the "old" toner solution present in the roll's spiral grooves. This pump assures that a constant supply of freshly circulated and mixed toner solution coats the surface of the roll.
Typically, this pump is approximately 5" in diameter and 12" long and this is fairly bulky. This pump draws a substantial current (about 3 amps in one system), produces a large amount of heat, and is relatively expensive and noisy. Also, connections to and from the pump require a substantial amount of large size tubing to circulate the toner solution in the circulating system. These large pumps have, in the past, required frequent service.
The toner solution is typically a factory prepared premix having 2% solids (i.e., 2% pigment) and 98% isopar (i.e., 98% clear carrier fluid).
In the prior art, toner solution is replenished as follows. As the paper to be printed is passed over the charging head the number of electrode locations (dots) energized for printing are counted. The amount of toner pigment used to develop (print) a given number of energized locations (dots) is known. Once a predetermined number of electrode locations (dots) have been energized, a predetermined metered amount of premix (having the same amount of pigment as was used to print the previously counted dots) is added to the circulating system as make-up to maintain the amount of toner pigment in the circulating system. However, during the printing process, the use of pigment and carrier in the toner solution is not uniform. The use, and resulting loss, of the carrier or isopar fluid when compared to the use, and loss, of toner pigment for a particular image, generally, does not match the ratio of carrier or isopar fluid to pigment in the pre-mix make-up solution (i.e., a relatively blank image uses more isopar than average, while a full image uses less isopar than average). Historically, this mismatched use eventually depletes the isopar carrier faster than the pigment is depleted thereby causing the concentration of pigment in the toner solution to increase. The concentration increases until the image developed on the paper becomes distorted and is not of good quality. The remaining toner solution, 1/3 to 1/2 of its original volume, is now judged to be unfit for further use and must be discarded and a fresh new supply substituted. Another problem is that the substantial amount of toner solution being discarded should be disposed of as hazardous waste.
Also, the prior art toner system requires a large removable sliding drawer in order to facilitate removal of the large toner bottles.